Learn about the different girth welding processes, when they should be employed and several other related aspects to ensure proper execution.
What is a girth weld? Girth welds refer to the welding processes employed for connecting two pipes along their circumference. Several factors must be considered for this, including environmental aspects and ease of implementation. These processes are used to make circumferential welds in underground systems as well as piping networks.
Read on to understand more details concerning girth welding, such as various girth welding methods and conditions that ensure accurate results.
More about girth welds
To complete girth welds, the operator carries out several passes in order to create a sealed and perfect weld joint. In the beginning, the operator must first complete the most difficult part i.e. the root pass. The root pass requires a specific speed for reliable quality. The next process is the hot pass, which adds upon the thickness of the joint created by the root pass. The final part is the fill and cap pass, which covers the weld joint to complete it. Furthermore, tie-in welding is an important facet of girth welding.
The following welding methods may be used for girth welding.
Manual arc welding
Shielded metal arc welding and gas tungsten arc welding are commonly employed.
Semi-automatic arc welding
Submerged arc welding, flux-cored arc welding, and gas metal arc welding.
Automatic arc welding
Laser-beam welding, flash-butt welding, and friction welding.
The welding methods, as well as the standards for girth welding, depending on the following issues.
- The base material strength of the pipes
- External conditions
- The pipe manufacturing process employed
- Pipe diameter and wall thickness
- Length and cost of the pipeline
- Terrain
- Welder skills
Tie-in – a key component of girth welding
Tie-in is a key aspect of girth welding. It refers to the connection of the pipeline with the facility of other piping networks. This term can also refer to the connections between different sections within a pipeline. It can also mean the modifications made to the existing piping systems. For instance, insertion of valves, spool pieces, and tees.
The current tie-in techniques are based on welding methods that were developed around 4 decades back. These methods require significant welder skill and manual work.
Tie-ins are carried out on the pipeline within its trench. The weld joint must be created between two completed piping sections. Hence, it is not practical to introduce new equipment within the pipe. To get around this issue, operations are executed externally. Therefore, the alignment, preparation and accuracy level of the pipe sections are crucial for reliable and strong weld joints.
External alignment clamps may be employed, although they reduce opportunities for automated welding processes. Experts from the pipeline construction industry largely agree that the tie-in procedure sees several benefits with the inclusion of automated welding. As of now, the process is mainly manual in nature.
The manual aspect of the tie-in procedure represents a major constraint towards pipeline welding productivity. Increased automation can drive down costs and improve productivity.
Currently, conventional construction techniques entail a tie-in after the crossing is reached at regular intervals. The tie-in procedure relies on welder skill and productivity, which can have a significant impact on cost and efficiency. This is easily evident in semi-urban as well as urban environments. It must be kept in mind that the rural regions also have several natural features, which can impede the progress rate.
Girth welding processes
The girth welding process is also required for repair welding as well as mainline welding. Compared to other welding processes, girth welding is far more challenging not only because of the high-level expertise required to weld pipelines, but also the adverse environmental factors that can hinder welding.
Several processes are utilized for girth welding. Here are the relative benefits and disadvantages of different girth welding processes.
Shielded metal arc welding
The key advantages of SMAW are that the equipment is simple and highly portable. This makes stick welding suitable for outdoor environments where the equipment may need to be hauled over uneven and difficult terrain.
No external flux or gas shielding is required because stick welding electrodes have flux coating, which generates both flux and shielding gas. Hence, stick welding is effective even in windy conditions.
There is also a large pool of available expertise since this is often one of the first welding processes that welding professionals learn.
However, its disadvantages should also be considered. Travel speed is low; hence, productivity is not that high. The fume generation rate is also very high, making this a hazardous process for welders. Welding operators must, therefore, don protective gear to shield themselves against noxious fumes.
Gas metal arc welding
The productivity rate of GMAW is one of the very best. This is due to the high deposition efficiency. Almost 90 to 97 percent of the welding wire turns to filler metal in GMAW.
Unlike stick welding, no slag needs to be chipped off after welding. This is another reason for high productivity.
The fume generation is low. Hence, it is a safer process for welding professionals. Owing to low hydrogen production, there is a less chance for hydrogen embrittlement and crack development.
What’s more, SMAW is easy to learn and has a good travel rate. These are further reasons for high productivity.
There are some disadvantages as well. SMAW requires tighter control over welding variables. Welding operators must also pay attention to good techniques; otherwise, there may be a lack of fusion. The equipment is also more expensive compared to stick welding.
Flux-cored arc welding
This welding process is quite similar to SMAW. The main difference between the two is that flux-cored arc welding has flux material in its center, as its name suggests. There is no need for external shielding gas since the core produces a protective gas layer.
The deposition efficiency is fairly high at over 80 percent. FCAW is also suitable for difficult welding positions. It is comparatively better than GMAW when it comes to managing rust and mill scale.
The key disadvantage is that the fume generation rate is high, which makes it a hazardous process for welding operators. Due to slag formation, chipping and finishing must be carried out in between passes, which can impact productivity.
High winds can also blow away the shielding gas and cause porosity. There are also few options for low hydrogen generation. Hence, there is the risk of cracks and holes in the weld due to hydrogen entrapment.
Submerged arc welding
The main advantage of submerged arc welding is that it produces fewer fumes. There is also minimal arc radiation. These factors make submerged arc welding relatively safe for welding operators.
The deposition efficiency is very high at over 95 percent. Hence, the productivity is high in this welding process.
Since this is a submerged welding process, there is no chance for drafts and winds, which can produce welding defects by blowing away the protective gas layer. As a result, the welds created are often strong and accurate.
However, certain disadvantages do exist. Since there is no visible arc, the filler material deposition is more difficult to follow. It has a fairly limited scope as it can be performed only for horizontal and flat positions. The initial investment is also quite high due to motion control and expensive welding equipment.
Gas tungsten arc welding
GTAW is often the best solution for precise welds and critical joints due to superior welding quality. It produces exceptionally strong and reliable welds with the added bonus that they have a good appearance.
Unfortunately, there are also serious drawbacks as well. Travel speed and the deposition rates are low, thereby leading to low productivity.
The UV rays are also much stronger compared to other processes. Hence, welding operators must ensure that they have proper welding helmets to filter out powerful UV rays.
The equipment cost is also higher. This means a higher initial investment. Operating costs can also be high, depending on the material being welded.
There are also other factors to consider when deciding the right girth welding process.
Thanks to advances in metallurgy, the industry is using pipelines made from steel with a minimum tensile strength of 56,000 psi. This comprises mostly of micro-alloyed steel.
Although the strength of the pipes increases significantly with micro-alloying, there is an increased risk of hydrogen-induced cracking. There is a higher risk for this along the heat-affected zone. To alleviate this issue, the X70 material grade pipes should be pre-heated to a temperature of about 140 degrees Celsius and cellulosic electrodes should be employed as well.